Spacefilling representation

Spacefilling representation is one way to represent a 3D structure. In spacefilling representation, each atom is depicted as a solid sphere, usually with a radius equal to some interpretation of its atomic radius. Viewing a molecule in spacefilling representation can crudely be thought of as a way to view the space that the molecule occupies.

On the right you see a n-lobe mutant of human transferrin (PDB code: 1dtg) in spacefill representation.